Facultative anaerobes, such as Escherichia coli, possess a genetic regulatory network to capitalize on the best mode of energy extraction under different environments. The most efficient mode is aerobic respiration. Expression of many genes in this process is controlled by a two-component regulatory system in which ArcB (the membrane sensor protein) communicates the signal to ArcA (the cytoplasmic response regulator) by phosphorylation. ArcA-P in turn regulates the expression of its target promoters. The family of target operons includes those that encode numerous primary dehydrogenases of the flavoprotein class, members of the citric acid cycle, and the aerobic electron transport chain. We propose to continue our investigation of the Arc system, focussing on the following problems. l) We will focus on the plasma membrane both as a source of a signal and a medium for transmitting the information to ArcB. 2) We will analyze the catalytic interactions between the various domains of the two regulatory proteins and the physiological significance of the structures. 3) The mechanisms by which ArcA-P represses the negatively controlled sdh operon (encoding succinate dehydrogenase) and activates the positively controlled cyd operon (encoding 02 scavenging cytochrome d) will be characterized. In particular we wish to discover the DNA consensus sequence in the target promoters recognized by ArcA-P (i.e., to determine the "Arc Box"). 4) Studies on another two-component system, Cpx, will be continued, because there is reason to think that its function is also related to regulation of membrane bioenergetics. In addition, the Cpx system might have some important role in membrane organization.